Electric discharge device



Patented Feb. 3, 1953 ELECTRIC DISCHARGE DEVICE Gerhard Lewin, Maplewood, N. L, assignor, by mesne assignments, to the United States of America. as represented by the Secretary of the Navy No Drawing. Application June 29, 1951, Serial No. 234,423

9 Claims. 1

This invention relates to electric discharge devices employing mercury pools or the equivalent for facilitating ionic conduction inside an enclosure. 1

The use of mercury pools as an element in an electrical devic such as an ignitron is well known, the pool functioning to maintain a discharge after initiation thereof. A disadvantage of mercury pools is inherent in the liquidity of the pool; namely, the distortion and destruction of function where the device containing the pool is subject to pronounced and erratic movement, as for example, an apparatus placed in a railroad train, on shipboard and other mobile uses. To overcome this difiiculty, it has been proposed to use metal sponges such as iron, but while iron readily absorbs mercury, it is not a satisfactory arc electrode. This deficiency may be met in part by placing a molybdenum insert in an iron sponge, holding mercury, the molybdenum having excellent arcing characteristics. However, this is not completely satisfactory, since as ordinarily treated molybdenum does not absorb mercury and the difliculties of securing adequate contact between the molybdenum and iron and the disadvantage of use of inserts generally calls for improvement.

Accordingly, the primary object of the invention is to provide a molybdenum electrode which contains appreciable quantities of absorbed mercury, and, in connection therewith, to provide a method of treatment of molybdenum which will facilitate entry of liquid mercury into the internal pore structure of the molybdenum.

In the known method of forcing mercury into iron sponge, the metal is exposed at about 750 C. for an hour to dry hydrogen, and then, while still in the hydrogen atmosphere, brought into contact with liquid mercury. The iron under these conditions absorbs the mercury into the mass pores to such an extent as to make the sponge satisfactory for such electrode uses as the mercury pool itself is usable. Subsequent heating of the mercury-containing sponge, as in boiling mercury, will increase the amount of mercury absorption by the iron to approach to the theoretical maximum of input.

In the case of molybdenum sponge, however, an equivalent treatment with mercury as above described for iron will, it effective at all, merely wet the surface of the molybdenum without any appreciable absorption by the internal pores of the sponge. It has been found, however, that the following treatment will produce a free mercury penetration of the molybdenum sponge. The

sponge is first heated for about an hour at 750 C. in an atmosphere of dry hydrogen at a pressure of about 760 mm. of mercury. By this step the oxide and adsorbed oxygen is removed from the molybdenum surface and replaced by a hydrogen film.

While the dry hydrogen is being maintained at the end of the one hour period, the molybdenum is immersed in boiling mercury and the boiling continued for from three to five minutes. During three to five intervals of three or four, and not over ten, seconds, each in this period, the hydrogen pressure is reduced from 760 mm. to a value between 500 mm. and 600 mm. For example, during a three minute boiling period there may be three pressure reductions separated by one minute intervals. During this pressure change the mercury boils violently with the effect that th hydrogen film on the molybdenum is removed and the metal is wetted throughout the pore surface with mercury. Care is taken that the low pressure, as 500 mm. of the hydrogen, is not continued too long at this value, as the mercury becomes rapidly cooled, the cleaning resulting primarily from the change of pressure and not from the absolute value of the pressure. A single pressure reduction is efiective to cause general penetration of the metal pores but multiple reductions are found most eflicacious. The method is certain and constitutes an adequate solution of mercury absorption by molybdenum sponge.

While the theory involved is not completely understood, it may be related to a relatively high adhesion of the hydrogen to the molybdenum thus normally preventing displacement of the hydrogen by the mercury. The sudden pressure drop gives the effect of super-heat, inducing violent boiling of the mercury and driving the hydrogen out of the sponge voids, so that the replacement by the mercury immediately results.

The application of the process to molybdenum has been described, but it is apparent that the process may be readily applied to other metals in sponge form which are not readily wetted by mercury, such as ruthenium, iridium, platinum, and palladium, and hence no limitation is implied except as may be required by the claims as hereto appended.

I claim:

1. A method of impregnating sponge metal with mercury which comprises boiling the metal in a bath of mercury and an atmosphere of dry hydrogen and, while the mercury is boiling, reducing the hydrogen pressure rapidly for e limited time period, until appreciable absorption of the mercury by the metal has been completed.

2. A method of impregnating sponge metal with mercury which comprises boiling the metal in a bath of mercury and an atmosphere of hydrogen for a time period of at least three minutes, and while the mercury is boiling, suddenly reducing the hydrogen pressure until appreciable absorption of the mercury by the metal has been completed.

3. A method of impregnating molybdenum metal sponge with mercury which comprises boiling the metal in mercury in 'anatmosphere of dry hydrogen for a time period of at least three minutes and while the mercury is boiling, suddenly reducing the hydrogen pressure for a time period of about three to five seconds.

4. A method of impregnating molybdenum metal sponge with mercury which comprises boiling the metal in mercury in an atmosphere of dry hydrogen for a time period of .at least three min- .utes, and While the mercury .-is .boiling, suddenly reducing the hydrogenpressure for a time period of about three to five seconds, restoring the hydrogen pressure to the initial value for a time period of about one minute and again suddenly reducing the pressure for a time period of around three to five seconds.

:5. -A method of impregnating metal sponge with mercury comprising the steps of heating the sponge in dry hydrogen gas at 760 mm. pressure at around 750 C. for about one hour, applying boiling mercury to said sponge while maintaining said pressure, suddenly reducing the. hydrogen pressure to a valuein the 500-600 mm. range and again restoring the pressure to 760 mm. in a time interval preferably within seconds.

.6. A method of impregnating metal sponge with mercury comprising the steps of heating the sponge in a dry hydrogen gas at 760 mm. pressure at around 750 C. for about one hour, applying boiling mercury to said sponge while maintaining said pressure, suddenly reducing the hydrogen pressure to a value in the 500-600 mm. range and again restoring the pressure to 760 mm. in a time interval preferably within 10 seconds, and repeating said pressure reduction and restoration while the mercury boiling is continuing.

7. A method of impregnating metal sponge with mercury'comprising the steps of heating the sponge in dry hydrogen gas at 760 mm. at around 750 C. for about one hour, applying boiling mercury to said sponge for about three to five minutes *while maintaining said hydrogen pressure, and during the period of mercury boiling repeatedly reducing the hydrogen pressure below 760 mm. to values. between 500 and 600 mm. for short time intervals preferably under ten seconds.

8. A method of impregnating metal sponge with mercury comprising the steps of heating the sponge metal with dry hydrogen gas to remove-oxide and adsorbed oxygen, immersing the sponge in boiling mercury at normal atmospheric pressure, and suddenly reducing the pressure to induce violent boiling of the mercury whereby the pores :of the metal are penetrated by the mercury.

9. A method of impregnating metal sponge with meroury-compirsing the steps of heating the sponge metal with dry hydrogen gas to remove oxide and adsorbed oxygen, immersing the sponge in boiling mercury at normal atmospheric pressure of 760 mm. and repeatedly reducing and restoring the hydrogen pressureto produce variable boiling activity of the mercury.

GERHARD LEWIN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,128,861 T'onks. Aug. 30, 1938 2,218,386 Smith e Oct. 15, 1940 

1. A METHOD OF IMPREGNATING SPONGE METAL WITH MERCURY WHICH COMPRISES BOILING THE METAL IN A BATH OF MERCURY AND AN ATMOSPHERE OF DRY HYDROGEN AND, WHILE THE MERCURY IS BOILING, REDUCING THE HYDROGEN PRESSURE RAPIDLY FOR A LIMITED TIME PERIOD, UNTIL APPRECIABLE ABSORPTION OF THE MERCURY BY THE METAL HAS BEEN COMPLETED. 